1. Field of Invention
The present invention relates to a method for cleaning a semiconductor wafer. More particularly, the present invention relates to a method for cleaning a semiconductol wafer that uses multiple baths and requires the minimum number of low temperature, low concentration chemicals to carry out the cleaning operation.
2. Description of Related Art
In the manufacturing of integrated circuit devices on a silicon wafer, cleaning operations are normally carried out both before and after a chemical operation, for example, a chemical vapor deposition, a dry or a wet etching operation. The purpose of such cleaning is to remove any organic or inorganic particles that are attached to the wafer surface. As the level of integration continues to increase, size of each device has shrunk to just a few microns. Therefore, any particles left on an unclean wafer surface can tremendously affect the finish quality in subsequent operations. If wafer cleaning operations are carried out inappropriately, the following problems may emerge:
1. Surface roughness of a silicon wafer may increase. Subsequently, when an oxide layer is formed over a silicon substrate, voids or crevices may form at the interface between the two layers. These voids trap and accumulate charged particles quite readily, leading to the formation of high-density residual charges (Dit) at the interface.
2. Residual impurities may also be attached to the surface of the wafer or trapped similarly inside the voids. These residual impurities can be quite easily transformed into charged ions. These charged ions diffuse inside the oxide layer, generating mobile charges (Qm).
The appearance of residual charges (Dit) and mobile charges (Qm) not only increase the leakage current, but also lead to the lowering of both the breakdown voltage (Vbd) and the breakdown charge (Qbd).
3. Any residual iron (Fe) or nickel (Ni) particles on the wafer surface tends to increase the amount of metal inside the gate oxide layer above. An increase in the metal content within the gate oxide layer leads to the lowering of surface recombination lifetime (SRL) as well as the bulk recombination lifetime (BRL). Moreover, the gate-oxide breakdown voltage (BvGox) of the gate oxide layer are lowered as well.
Consequently, if a given wafer cleaning process retains some of the unwanted particles on the surface, quality and reliability of the finished semiconductor devices may deteriorate.
FIG. 1 is a flow diagram showing the series of steps in a conventional multi-bath wafer cleaning method that prepares the wafer surface for gate oxide deposition. Here, a multiple-bath station is used. First, the silicon wafer is placed inside a dilute hydrofluoric acid (DHF) bath 110 so that sacrificial oxide above the wafer is removed. Then, the wafer is transferred to an overflow (OF) bath 112 to remove any residual DHF acid on the wafer. Thereafter, the wafer is washed inside an RCA1 cleaning bath 114 using RCA1 solution. The RCA1 solution is a mixture containing ammonium hydroxide (NH.sub.4 OH), de-onized water (HDIW) and hydrogen peroxide (H.sub.2 O.sub.2). The RCA1 solution is heated to about 40.degree. C. to 70.degree. C. to remove organic particles from the surface of the wafer.
Next, the silicon wafer is placed in a hot, quick-dump rinse (HQDR) bath 116 to remove residual RCA1 solution using a large quantity of de-ionized water. Then, the wafer is placed inside another dilute hydrofluoric acid (DHF) bath 118 to remove native oxide.
Thereafter, the wafer is placed inside another overflow (OF) bath 120 to remove any residual DHF acid. Then, the wafer is washed in an RCA2 cleaning bath 122 using RCA2 solution. The RCA2 solution is a mixture containing hydrochloric acid (HCl), hot de-ionized water (HDIW) and hydrogen peroxide (H.sub.2 O.sub.2). The RCA2 solution is heated to about 40.degree. C. to 80.degree. C. to remove metallic impurities and particles from the surface of the wafer.
In the subsequent step, the wafer is placed inside another HQDR bath 124 to remove residual RCA2 solution using a large quantity of de-ionized water. Next, the wafer is placed inside a final rinse (FR) bath 126 wherein the wafer is further cleaned.
Finally, the wafer is placed inside a de-moisturizing bath 128 containing isopropyl alcohol (IPA). Inside the bath 128, moisture on the wafer surface is carried away by the IPA vapor, thereby obtaining a clean, dry surface.
In the above-mentioned method, a batch of silicon wafers are moved from one cleaning bath to another until the whole cleaning process is complete. The advantage of such method is the capacity for each bath to work independently. If any one of the baths is polluted, only the batch of wafers in that particular batch are affected. Therefore, the problem bath can be fixed without having to shut down the whole cleaning station. However, the entire cleaning operation involves many steps (a total of ten as shown in FIG. 1 ) and each step requires a cleaning bath hence a lot of space is needed to accommodate these baths. Furthermore, temperature and concentration of the chemical solution in these baths are generally high, and the kind of chemicals employed in the cleaning process are many. Consequently, a large sum of money has to be spent on the design and maintenance of the cleaning station not to mention the charges for polluting the environment and other safety considerations. To minimize the defects of a multi-bath design, an improved cleaning station using just a single bath has been developed.
FIG. 2 is a flow diagram showing the series of steps carried out in a single bath wafer cleaning station. Here, only a single cleaning bath 210 is used. All the cleaning steps are carried out within this cleaning bath 210. First, the silicon wafer is placed inside the cleaning bath 210 and then dilute hydrofluoric (DHF) acid is poured into the bath 210 for removing sacrificial layers above the wafer. Thereafter, de-ionized water is pumped into the bath 210, and then the de-ionized water is permitted to overflow so that any residual DHF acid is carried away. Next, RCA1 solution is pumped into the bath 210 to clear away any organic particles on the surface of the wafer. In the subsequent step, using a hot, quick-dump rinse (HQDR) method, a large quantity of de-ionized water is pumped into the bath 210 for rapidly removing residual RCA1 solution on the wafer. After that, ozone (O.sub.3) is passed through the bath to remove organic or metallic impurities. Next, dilute hydrofluoric (DHF) acid may be added to the bath 210 if native oxide needs to be removed from the wafer, as well.
Thereafter, an overflow (OF) method using de-ionized water is carried out to remove any residual DHF acid on the wafer. Then, a final rinse is conducted to clean the wafer further. Finally, isopropyl alcohol (IPA) is pumped into the bath 210 so that the IPA vapor can carry any moisture away, further drying the wafer.
The single bath method for wafer cleaning has several advantages. Firstly, the cleaning station occupies a rather small foot-print. Secondly, design and material cost of such a station is smaller. Thirdly, in each cleaning operation, fresh, new, chemical solution is pumped in and hence the process is able to maintain a high level of chemical activity. In addition, proper cleaning can be achieved with a somewhat low temperature, low concentration, chemical solution.
However, since fresh chemical solution has to be added at the start of each cleaning operation, a lot of chemicals and de-ionized water are wasted. Moreover, batch mode of production is employed in a single bath cleaning station. In other words, a batch of silicon wafers is put inside the single bath to carry out the series of chemical cleaning operations. The batch of wafers remains inside the bath until the series of cleaning steps have all been completed. Therefore, there is very little tolerance for errors because the entire batch of wafers may have to be scrapped. Furthermore, as the number of steps required for cleaning a batch of wafers increases, operation of the cleaning station is too complicated to lose throughput very much. Moreover, if the bath is polluted, the entire cleaning station has to be shut down, adding difficulties in recovery the entire system.
In light of the foregoing there is a need to improve the method for cleaning semiconductor wafers.